This article was originally published in the March/April 1994 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.

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Home Energy Magazine Online March/April 1994

THERMAL COMFORT

Some Like It Hot

by Alan Meier

Alan Meier is executive editor of Home Energy

This article, the second in Home Energy's tenth-anniversary series, originally appeared in 1986. It offers tips on how to keep warm in the winter while still conserving energy.

Almost everybody knows when they are cold, but it is difficult to predict an individual's thermal comfort for a particular set of conditions. For example, would somebody be comfortable wearing cotton pajamas in a 64deg.F room while assembling an intricate jigsaw puzzle? Such questions may seem trivial for everyday activities, where adjusting the thermostat or putting on a sweater are simple alternatives, but predicting thermal comfort is critical for astronauts, arctic oil drillers, and a scuba divers. In less esoteric situations, such as indoor swimming pools, gymnasiums, auditoriums, and environmentally controlled clean rooms, thermal comfort is an important design consideration.

A science based on quantifiable environmental parameters has evolved to predict thermal comfort. Thermal comfort science permits auditors and retrofitters to recognize comfort problems and propose more creative solutions.

What is Thermal Comfort?

The human body burns food and produces a significant amount of heat. A sleeping adult generates about the same amount of heat as a 100 W light bulb. An athlete at peak exertion generates over 1,000 W--about the output of a portable space heater or hair dryer. If the body cannot easily rid itself of the heat--be it 100 or 1,000 W--the physiological message sent to the brain is, I'm hot! Overheating occurs because the body is unable to dispose of heat as fast as it is produced. In cool situations, the rate of heat loss to the environment exceeds the body's heat production (I'm cold!).

While some heat is removed through breathing, heat loss through the skin is by far the major path. Cold receptors in the skin signal the brain if the temperature on the skin (or a limited area on the skin) drops at a rate faster than 0.5deg.F per minute. The appearance of a cool draft, which can change local air temperatures by 2deg.F in a matter of seconds, easily exceeds this threshold.

The human body has evolved a remarkable thermoregulatory system. One response is to increase thermal output to match the loss. The muscle tensions that cause shivering, for example, can cause a three-fold increase in heat production. At the same time, the body tries to cut heat loss by limiting blood flow to the skin and extremities. A colder skin reduces heat loss. Skin temperatures are generally well below 98deg.F at room temperature. The mean skin temperature of a nude person at room temperature is only about 90deg.F. Feet temperatures drop to less than 80deg. F!

In cold situations, the blood vessels near the surface constrict. Vasoconstriction generally occurs in the extremities first, that is, the fingers, hands, feet, nose, and so on. Once the blood vessels are fully constricted, the skin acts similarly to clothing, and provides useful insulation. Heat is also conducted from the warmer core to the surface through the skin. Its insulation value depends on the thickness of the fat layer.

The Fundamental Trade-off

The familiar heat loss equation, Q = UA(Tin-Tout), governs the major heat loss paths in a house. Most conservation measures try to reduce the rate of heat loss, Q, by lowering the U-value, that is, increasing the thermal resistance of the walls, ceiling, and windows. A few conservation strategies try to reduce the surface area, A.

Another method of reducing heat loss is to reduce the temperature difference. We can only affect the temperature inside the house, since the outside temperature stubbornly refuses to bend to our will. A setback thermostat saves energy but results in a colder, less comfortable home. But the intelligent application of thermal comfort research permits occupant comfort at a lower air temperature by improving other environmental parameters. (See Figure 1) These measures rarely permit more than an 8deg.F offset, but even a 5deg.F drop in thermostat setting over the whole winter can significantly lower the heating bill--often by as much as 15%. In other cases, thermal comfort measures might be used to make a home more comfortable without turning up the thermostat. Such measures correspond to shifting to a higher line of thermal comfort in Figure 1.

Elements of Thermal Comfort

Most people wrongly equate thermal comfort with the temperature. In fact, air temperature is only one of many factors affecting comfort. The major factors can be viewed as personal (the insulation value of clothing, activity level) and environmental (air temperature, mean radiant temperature of surrounding surfaces, air speed, humidity).

Clothes Make the Man (Warm)

The clo-value of clothing closely corresponds to the R-value of fiberglass and other building materials. Researchers in Denmark and the United States have measured the clo-values of different clothing ensembles. By definition, a nude person has a clo-value of 0.0, while a typical business suit has a clo-value of 1.0. An Eskimo's cold-weather parka has a clo-value of about 4.0. Table 1 shows the clo-values for different items of clothing and ensembles. There are also estimates of clo-values of animals' skins. The tiny shrew's fur coat provides only a clo-0.3 whereas the polar bear's coat is about clo-5. The relationship between clo-value and thermal comfort (for humans) is shown in Figure 2.

Recommending that occupants wear more clothes is not always a good idea, because people may see wearing warmer clothing as an unacceptable sacrifice. But specific items of clothing, such as socks or a cap, may be accepted. Even small increases in the occupants' clo-values can permit significant reductions in the thermostat setting-as much as a few degrees--especially if the thermostat-setter (or chief complainer) is a little warmer.

Keep Active!

One way to warm up is to increase heat output or, simply, to keep active! The level of activity is measured in units of mets (short for metabolic units). One met is defined as the rate of heat produced when a person is quietly seated, and it corresponds to about 58 W per square meter of body surface area. Since adults range in surface area from 1.5 to 2.0 square meters, the heat produced by a sedentary adult is 90-115 Watts. Heavy work corresponds to about 3 mets, but an athlete can achieve (briefly) 9 mets. Table 2 lists the met-values for several activities.

Even a little activity can improve thermal comfort. Table 2 shows that switching from quietly sitting to standing can increase heat output 20%, which is often sufficient to re-establish thermal comfort. Strenuous exercise provides so much more heat that one can overheat even in cold conditions. (This explains why careful consideration of thermal comfort is needed in gymnasiums and swimming pools.)

Differing activity levels often explain different thermostat preferences. For example, it is commonly believed that older people need higher temperatures to keep comfortable. Research has shown that age does not significantly affect thermal comfort preferences. However, older people are generally less active, and the lower activity level requires a higher temperature to maintain comfort.

When evaluating the feasibility of a thermostat setback, it is essential to consider the activity levels of the occupants, especially if there are elderly persons in the house. Recommending periodic calisthenics may improve their health but you may also get you laughed out of the house. Still, people sometimes forget that even a little activity can vastly increase their comfort.

The next group of thermal comfort factors are externally, rather than personally, generated. These factors--air movement, mean radiant temperature, and humi- dity--offer many retrofit opportunities. Again, the primary goal is to reduce the air temperature (lower thermostat) while maintaining the level of thermal comfort by increasing other thermal comfort factors. In other cases, the goal is to increase thermal comfort without raising the thermostat setting.

Avoiding the Draft

Air moving over the skin (or clothes) robs the body of heat. The source of air movement may be natural convection, a fan or draft (forced convection), or body movement caused by walking or running. In these situations, higher air temperatures are necessary to maintain the same thermal comfort level (See Figure 3). People are very sensitive to changes in air speed, and as Figure 3 shows, air movement can change the comfort temperature as much as 5deg.F under typical domestic conditions. The peculiar elbows in the comfort curves in Figure 3 reflect the destruction of the body's surface air film. As air speeds increase above 0.4 feet per second, the surface air film collapses and ceases to provide insulation. Heat loss increases markedly, which is reflected in the rapid increase in the comfort temperature. Accurately measuring low air speeds is expensive, but rough measures are possible. A draft exceeds 0.5 feet per second. A hand-operated fan, held a few feet away, can create local air speeds exceeding 1.2 feet per second.

House-tightening measures can significantly reduce air movement in a house. The blower door is an effective tool in identifying leaks. Note that house-tightening (that is, weatherstripping, leak-plugging, and other infiltration reducing measures) saves energy two ways. Since it reduces air infiltration, less air needs to be heated. Of secondary importance, the slower air movement permits a lower thermostat setting. (Unfortunately, it is impossible to translate reduced infiltration into lower air movement rates because infiltration is so uneven.) Upon completion of a major house-tightening, the retrofitter should recommend a 1-3deg.F thermostat setback if the occupant wants to achieve maximum savings. On the other hand, if occupants want a combination of savings and increased comfort, the thermostat can stay at the original temperature.

Sometimes drafts are inevitable. For example, houses without vestibules suffer from a blast of cold air (over 6 feet per second) each time an outside door is opened. Even warm air moving at a high speed feels uncomfortable. Heat pumps need high air flow through the ducts, and this often causes drafts near their registers. In these situations, repositioning furniture, or possibly relocating the most frequent activities, can lead to greater comfort. Deflectors on heating registers are simple and effective. Sites of sedentary activities, such as television-watching or card-playing, should be carefully assessed. Keep the favorite television chair away from drafts!

Radiant Temperature

All objects lose heat through radiation. The sun's radiation is the most familiar example. A person immediately senses the radiation when walking out of the cool shade and into the sunlight. Many new bathrooms are equipped with infrared lights to provide quick radiative heating. Radiant temperatures in a building are an important, but subtle, factor in determining thermal comfort.

Radiative heat loss is totally unlike the familiar conductive heat loss. The rate of radiative heat loss depends on the object's temperature. At the same time, all the surfaces around an object are radiating heat towards the object. The object's net radiative loss is therefore the difference between the heat it loses (a function of the object's temperature) and the heat it gains (a function of the surrounding surfaces' temperatures). Radiation is subtle because radiative heat loss is essentially independent of the air temperature around the object. People radiate heat, too, although the rate is much less intense than the sun (because a human's temperature is only about 100deg.F).

The mean radiant temperature (MRT)--the area-weighted average radiant temperature of the surfaces surrounding the person--is one factor determining thermal comfort. The combination of a very hot radiant source having only a small area (such as a quartz radiant heater) and large, cool walls can yield an acceptable average radiant temperature. The relationship between the mean radiant temperature and the air temperature is shown in Figure 4. There is a direct tradeoff between MRT and air temperature. A constant comfort level can be approximately maintained by raising the MRT 1deg.F and setting back the thermostat 1deg.F. Many people sense that the room is more comfortable after increasing the radiant temperature, but they are unable to attribute it to a particular difference. A room with high radiant temperatures simply feels more comfortable. Others comment that the temperature seems more constant.

To increase the mean radiant temperature in a room, eliminate cold surfaces. The window is the first culprit (if no sun is shining though it). Windows make a disproportionately large contribution to a low mean radiant temperature because they are so cold. Somebody sitting near a window on a 0deg.F winter day will need a 9deg.F higher air temperature to maintain thermal comfort. (lf unable to turn up the heat, most people move, put on warmer clothes or complain.) The temperature of a single-glazed window's inside surface--that's the one that counts--will be less than halfway between the outside and inside temperature.

There are many ways to increase a window's radiant temperature. The cheapest retrofit is a heavy curtain or a piece of rigid foam insulation (which is more effective). A heavy curtain shields the cold window and presents a new interior surface that is much closer to room temperature. Installing a double-glazed or storm window will raise the surface temperature so that it is about 5/8 of the outside-inside temperature difference. Triple-glazed windows present an even warmer inside surface. Of course, one should make the north-facing windows the first target of any retrofit.

An invisible benefit of insulating the ceiling and walls is an increase in radiant temperature. The increase may be relatively small compared to window improvements, but the large area affected results in a significant increase in mean radiant temperature--sometimes worth a few degrees of air temperature. Again, occupants often indicate that the house feels warmer, or more evenly heated, after insulating.

Installing radiant heating ceiling panels is another way of increasing a room's MRT. These panels contain wires heated by electrical resistance. The manufacturers argue that lower inside air temperatures are possible with radiant heating, so radiant panels should should be considered a conservation measure. (They typically contend that the higher MRT permits a 5deg.F lower air temperature.) The argument has some merit but the heat may still be more expensive because it is electric resistance and some heat may be lost upwards, into the ceiling. Another problem is that the untrained consumer may still put the thermostat at 70deg.F, so no energy is saved. If, however, radiant panels are installed in conjunction with a comfortstat, that is, a device that measures thermal comfort based on both air and radiant temperature, energy savings would be more certain.

It is difficult to calculate the savings, but any single action (wall insulation, ceiling insulation, or window upgrade) should permit a 2deg.F setback while maintaining constant thermal comfort.

Recommendations

Thermal comfort means much more than just a high thermostat setting. Comfort depends on the occupants' clothing and activity levels, and on a home's construction features, including air flow and mean radiant temperature. These factors can be modified through retrofits and careful recommendations to permit a lower thermostat setting and energy savings, while keeping occupants just as comfortable. Major retrofits, such as insulation, window treatments, and house-tightening, should be accompanied by thermostat setbacks of 1-5deg.F. Also, the location of activities inside the home should be examined from the perspective of drafts and low radiant temperatures. Even a modest repositioning of furniture or small retrofits can can vastly enhance thermal comfort. n

Figure 1. The potential for offsetting reductions in air temperature with increases in other parameters.Major parameters include insulation value of clothing, activity level, air movement, and mean radiant temperature of surfaces (although in this diagram they have been graphed on an arbitrary scale). Thermal comfort measures can also make a home more comfortable without increasing heating energy use. Such measures correspond to shifting to a higher line of thermal comfort.

Figure 2. The relationship between clo-value and thermal comfort. A person can maintain a constant level of thermal comfort at colder air temperatures by putting on more clothes. (An obvious concept, but for once it is quantified.) A nude person has clo-0.0 and a standard business suit is about clo-1.0.

Figure 3. The effect of air movement on comfort. A person needs higher air temperatures to maintain comfort when air moves past them. Air movement can change the comfort temperature as much as 5deg.F under typical domestic conditions. The elbows in the comfort curves reflect the destruction of the body's surface air film. As air speeds increase above 0.3 feet per second, the surface air film collapses and ceases to provide insulation. Heat loss increases markedly, which causes increased discomfort.

Figure 4. The relationship between the mean radiant temperature and air temperature. There is a direct tradeoff between MRT and air temperature. An approximately constant comfort level can be maintained by raising the MRT 1deg.F and setting back the thermostat 1deg.F.